Method of effecting gamma phase precipitation to produce a monocrystalline growth in permanent magnets



E. STEINORT Nov. 23, 1965 3,219,495 ODUCE A METHOD OF EFFECTING GAMMAPHASE PRECIPITATION TO FR MONOCRYSTALLINE GROWTH IN PERMANENT MAGNETSFiled April 5, 1965 2 Sheets-Sheet 1 ffiflfl 6------ I N VE N TOR.[EiZ/MED Spa/meg;

E. STEINORT Nov. 23, 1965 3,219,495 MMA PHASE PRECIPITATION TO PRO LINEGROWTH IN PERMANENT MAGNETS DUCE A METHOD OF EFFECTING GA MONOCRYSTALFiled April 5, 1963 2 Sheets-Sheet 2 v, w 6 5 M M W w w M flames I N VENTOR. fame 4w firm/oz;

yI/IIII United States Patent 3,219,495 METHOD OF EFFECTING GAMMA PHASEPRE- CIPITATION TO PRODUCE A MONOCRYSTAL- LINE GROWTH IN PERMANENTMAGNETS Eberhard Steinort, Milan, ltaly, assignor to Centro MagnetiPermanenti, S.p.A., Milan, Italy, a corporation of Italy Filed Apr. 5,1963, Ser. No. 276,684 Claims priority, application Italy, Apr. 6, H62,6,949/62 24 Claims. (Cl. 148-191) This invention is a modification andimprovement of the subject matter of my prior copending applicationSerial No. 14,348, filed March 11, 1960, now Patent No. 3,085,036, datedApril 9, 1963. This invention relates to a process of making permanentmagnets having a monocrystalline structure, and to certain productsproduced in such method.

It is the general object of the invention to provide a modified andimproved method of making monocrystalline magnets, which will be betteradapted for commercial use, will avoid the necessity for certainconditions and steps which are preferably used in the method disclosedin my prior application, and which will reliably produce monocrystallinestructure in originally polycrystalline magnet castings.

In the preferred method of said prior application, monocrystallinestructure is produced in normally polycrystalline magnet castings by (a)including in the magnet alloy a gamma-phase precipitant, (b) castingsuch alloy into molds to produce magnet castings which contain thegamma-phase precipitant throughout their entire bodies, (c) heattreating such gamma-phase-containing castings at a gamma-phaseprecipitating temperature, e.g. l0001050 C., to produce gamma-phaseprecipitation therein, and thereby produce critical strain conditionstherein, and (d) heat-treating the castings at a recrystallizingtemperature, e.g., 1260-l310 C., with gamma-phase precipitate presenttherein, for a time sulficient to permit monocrystal growth and toredissolve the gamma-phase precipitate. To secure monocrystalorientation relative to the preferred diection of magnetization in themagnets produced, the recrystallization heat-treatment is carried out ina manner to produce a temperature gradient from one end of the casting,so that the monocrystal growth is nucleated at such end and progressesin a direction normal to such end.

The presence of gamma-phase precipitate is highly undesirable infinished magnets because of its deleterious effeet on magneticcharacteristics, and after the recrystallization treatment in the priorprocess, it is necessary to cool the treated magnet bodies rapidlythrough the gammaphase precipitating range in order to avoid thepresence of gamma-phase precipitate in the magnets.

In accordance with the present invention, the magnet alloy from whichthe magnet bodies are cast contains no added gamma-phase precipitant andis of a normal composition selected solely for its magnetic and othercharacteristics. Instead of producing in the entire magnet body thecomposition and conditions needed to cause gammaphase precipitation andresulting critical strain, the present invention contemplates theproduction of such composition and conditions in only a thin layer at aselected face of the magnet casting. The monocrystal growth is startedin this layer and, once sufiiciently started therein, will progressthroughout the normal alloy composition of the main body of the casting.The thin layer in which the gamma-phase precipitate and strainconditions are produced is provided by enrichment with suitablecomponents, as will be explained below. Such layer may be removed in thenormal finishing operations on the magnets, so that the entire magnetproduced will be of the normal alloy composition.

Since the body of the magnet will not be of a composition which tends toproduce gamma-phase precipitate, special rapid cooling need not be used,and standard production procedures can be followed. Moreover, theenriched layer on a selected face of the magnet has the effect itself ofcontrolling the location of monocrystal nucleation and the direction ofmonocrystal growth, so that no particular temperature gradient need bemaintained during the recrystallization heat-treatment.

The present invention, like that of said prior application, isapplicable to the known high-strength iron-nickelaluminum type alloysbasically composed of iron, nickel and aluminum, and commonly includingcobalt, copper, and various addition elements. These comprise thealnico-type alloys containing 10 to 30 percent nickel, 6 to 14 percentaluminum, 5 to 42 percent cobalt, up to 8 percent copper, up to 10percent titanium, with the balance substantially all iron butpermissibly containing up to 5 percent of silicon, zirconium, columbium,or other known addition elements. The preferred alnico alloys contain 12to 20 percent nickel, 6 to 11 percent aluminum, 16 to 30 percent cobalt,2 to 6 percent copper, and from a trace to 7 percent titanium;

The prior application discloses that monocrystal growth is favored bythe presence of certain elements which widen the gamma-phase loop in theiron phase-diagram and which promote gamma-phase precipitation andstabilize gamma-phase precipitate. These were listed as includingcarbon, nitrogen, manganese, ruthenium, rhodium, rhenium, osmium,iridium, platinum, and gold. They also include cobalt and nickel whichin normal alnico alloys have their action suppressed by co-presentaluminum.

In alnico alloys these elements have the effect of displacing to theright the line representing that alloy in the phase diagram, and henceof enlarging both upward and downward the temperature range within whichgamma-phase is precipitated and is stable.

In the process of the prior application, the monocrystalline growthtakes place starting with crystals already present as part of theoriginally cast polycrystalline structure, but which have their axisparallel to What will subsequently be the direction of preferredmagnetic orientation. Such crystals occur at particular outer faces ofthe magnet castings. Accordingly, in the recrystallization treatment itis necessary to take care that such particular faces are heated in anespecially intensive way to develop a temperature gradient in thedirection of the (100) axis of such crystals, and thereby cause themonocrystalline growth to develop from such crystals and to progress inthat same direction. This need for a directed temperature gradient is aninconvenience.

Moreover, the addition and control of the gammaphase precipitants in theentire alloy in the prior process was somewhat critical, for if excessamounts were present, either from an original formulation or by reuse ofscrap, or if cooling from recrystallization temperature was notsufiiciently rapid in the magnet castings or in parts thereof, thedanger existed that gamma-phase precipitation would occur during suchcooling, and that gamma-phase precipitate would therefore be present inthe magnets and would adversely affect their magnetic properties.

Further studies and research about the mechanism of crystal growth inmagnet castings have brought about results which extend beyond thedisclosure of the prior application and permit decisive improvements.

It was possible to establish that, for granule growth, particularimportance resides in the energies existing at the granule or grainboundaries, which depend to a large extent on the internal stresseswithin the crystal grains, which in turn are susceptible to influence bythe shape or size of the granules and by the quantity of gamma-phaseprecipitate present.

Considerable difference of energy at the grain boundaries occur as theresult of the presence in the structure of the casting of grains oflarge size in direct proximity to grains of small size.

Now with respect to the volume of granules, those of small size haverelatively larger surface areas than those of larger size. Since thecrystal grain boundaries correspond to the external surfaces of thegranules, if equal magnitude of absolute energy in different crystalgrains is assumed, the grains of smaller dimensions will have highergrain-boundary energies per unit of area than the larger grains. In anysystem involving energy differences, the basic tendency is for change togo in the direction which gives the minimum value of total energy. Thus,in a crystalline structure, the trend is in the direction to relieve ordestroy the elevated energy levels existing at the grain boundaries, andis toward a condition in which there is no longer any energy at thegrain boundary, which is the condition of a monocrystal which has nograin boundary with any other crystal.

The elimination of the energy differences at the grain boundaries takesplace by means of displacements of the grain boundaries where thegranules of smaller size are disintegrated and absorbed by the adjacentgranules of larger size. That however is possible only by acorresponding mobility of the atoms, which occurs only at highertemperatures.

If a big granule has caused the disintegration and the absorption of asmaller one, then the first big granule, now still larger, comes intoproximity with another granule and again a difference occurs between theenergies existing at the grain boundaries, so that again the adjacentsmaller granule is absorbed. In consequence of such constant growth of agranule, the difference of the relative energies existing at theboundaries of said granules, being proportional to the granule volumedifferences, becomes greater and greater and the speed of growth of thegranule increases continuously. Accordingly, if monocrystal growth isonce started, it tends to proceed to completion.

The differences of volume of adjacent granules formed by normal alnicoalloys, cast normally, are not of sufficient magnitude to cause withcertainty the beginning of the first process of growth. Hence theformation of monocrystal structure in such alloys, in the absence of aheat-treatment to precipitate gamma-phase, takes place only incidentallyand only when by chance a very big granule and a very small granulehappen to be adjacent to each other, and where then the difference ofthe relative energies existing at the grain boundaries is higher than acritical value.

Now, by measurement of micro-hardness, it can be shown that gamma-phaseprecipitate in the basic alpha structure produces a substantial increaseof the microhardness of said alpha phase. Further it has been possibleto establish an inter-relationship between the microhardness of thebasic structure and internal stresses, and an inter-relationship betweenthe internal stresses and the absolute energy existing at the boundariesof the granules per unit of surface area. This may be explainedtheoretically by the fact that the alpha phase has a body-centered cubicstructure whilst the gamma-phase has a face-centered cubic structure andthat the two reticular structures present constant differences ofreticle and different coefficients of heat expansion.

If by means of gamma segregation the absolute values of the energies atthe boundaries of the granules are increased, then the process ofgranule growth to form a monocrystal will take place with certainty evenwith smaller diflerences of volume between adjacent granules. From thediagram of phases illustrated in my prior application, FIG. 1 in theaccompanying drawings, it can be seen that the region with two phasesAlpha+ gamma, for the standard alloy of composition: 24% Co, 14% Ni, 8%Al, 3% Cu and with about 0.015% of C extends only over an interval oftemperature approximately ranging from 30 C. to 1,200 C., where thegamma-phase again returns to solution at temperatures of from 1,175 C.to 1,200 C. However, since as already set forth, a certain mobility ofthe atoms is necessary for displacement of the grain boundaries, andthis is produced only above temperatures of 1,220 C. approximately, itis evident that it is necessary to keep the gamma-phase stable at highertemperatures. Otherwise, the gamma-phase becomes dissolved before thenecessary mobility of the atoms is attained. Such stabilization ispossible by addition of the elements mentioned in my co-pending PatentNo. 3,085,036, the use of which as alloying elements in the entiremagnet casting, however, involves certain sources of danger, as alreadyset forth.

On the basis of my discovery that the crystal growth continues withoutthe presence of gamma-phase precip itate, if the difference of volume ofadjacent granules once becomes sufficiently gerat, I have now foundaccording to the present invention that it suffices to start the growingprocess of the granules in a layer with stable gammaphase, and that tostart this process it sufiices to enrich a thin layer of the magnet withan element which produces a stable gamma-phase precipitate therein. Inthe layer containing stable gamma-phase precipitate, the process ofgranule growth can be reliably started, and a certain crystal in suchlayer will grow in the layer until it attains a size that is sufiicientto ensure continued progressive growth to disintegrate and absorb thegranules of the unenriched main body of the magnet, where, in theabsence of growth-stimulating gamma-phase precipitate, the granules haveremained small.

Accordingly, the process of the present invention is characterized inthat the body of the magnet is cast of standard or normal alloy, toproduce a crystal structure in which the polycrystalline grains are ofnormal small size and in which the occurrence of gamma-phase precipitatetends to be suppressed, in accordance with normal magnet producingpractices; and that the process of producing gamma-phase precipitate andstabilizing the same, as disclosed in said prior application, is appliedto only a thin layer of the magnet casting, in a thickness of the orderof from 0.5 to 1 mm. or so. This may be done by enriching the layer bythe addition to that layer of the known gamma-phase precipitants listedin said prior application, or by enriching the layer with the elementsnickel or cobalt, which have the same effect and which are alreadypresent in selected proportions for other purposes.

The action of the gamma-phase precipitants, especially that of nickeland cobalt, depends on the amount of aluminum present, since aluminumhas a gamma-phase suppressing effect. Accordingly, the desiredgammaphase-producing layer can be obtained both by adding a gamma-phaseprecipitant thereto, and by preferentially removing thegamma-phase-suppressing aluminum while leaving a gamma-phaseprecipitant, such as nickel and cobalt, which is already present.

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a reproduction of the phase diagram showing phase changes of atypical Alnico alloy;

FIG. 2 is a diagrammatic sectional view of a magnet casting at anintermediate stage of the process; and

FIG. 3 is a diagram of a heat-treatment cycle used in a process whereinaluminum is removed from a surface layer of a magnet casting, asdescribed in Example 9 below.

In the diagram of FIG. 1, the vertical dotted line rep-.

resents an Alnico alloy of the composition indicated. The diagram showsthat this alloy has a pure alpha structure both above 1200 C. as well asin the range of 900930 C. To obtain good magnetic properties, this alloymust be magnetically heat-treated, i.e., brought under the influence ofthe magnetic field While it is in the pure alphaphase state. Anymagnetic heat-treatment from temperatures between 930 and 1180" C. leadsto poor results because the structure is not pure alpha-phase in thistemperature range. At temperatures between 930 C. and about 1150 C. asecond or gamma-phase is precipitated, first at the grain boundaries andthen also within the crystals. From about 1175 C. to 1200 C. thegammaphase again enters into solution. The presence of this gamma-phasein a magnet casting during heat-treatment in a magnetic field has a verymarked deleterious efiect on the formation of a preferred magneticorientation. Even the presence of very small amounts of gamma-phase isenough to decrease the magnetic values to such a point that the magnetsare useless. A 7% content of gammaphase will lower energy values byabout Recent studies indicate that preferred magnetic orientation is theresult of the directional precipitation of submicroscopic particles of asecond alpha-phase, referred to as alpha-prime, at temperatures below900 C. However, the presence of even minor quantities of the gammaphaseobstructs the proper establishment of a preferred magnetic orientation.These gamma-phase precipitates also cause major lattice distortions withassociated large internal mechanical strains, and these serve to inducelarge-crystal growth under suitable recrystallization heat treatment.

FIG. 2 of the drawing represents a magnet casting 10 at an intermediatestage of the present process. The main body 12 of the casting,constituting substantially the entire casting, is in its originalas-cast condition. The magnet is assumed to have been cast from astandard magnet alloy such as an alnico alloy of the preferredcomposition given above, and to have been cast by standard castingprocedures. The main body 12 is represented as having the completelypolycrystalline and desirably fine-grained structure normally producedby such casting, and the granules are represented as having noparticular orientation, since the present invention requires none,although in practice the crystallites adjacent the surfaces which werecooled by contact with the mold would normally be oriented with their(100) axes normal to such surfaces. It is known that certain additionelements such as titanium produce fine-grained structure, and suchstructure is desirably present for purposes of the present invention.

The top and side surfaces of the casting 10 are coated with a protectivecoating 14 of a character commonly used in metal processing operationsto protect the underlying surface from a carburizing, nitriding, orother surface treatment. The bottom surface 16 of the casting isuncoated, and left exposed, and a thin layer 18 of the casting at thissurface is shown as modified from its as cast condition. In accordancewith the invention such a layer is produced at a selected face of themagnet casting, and in such layer the composition is enriched with agammaphase precipitant or otherwise modified to promote theprecipitation of gamma-phase and to stabilize the precipitate produced.As shown, such layer desirably has its crystallites oriented with their(100) axes in a predetermined relationship to the body of the casting,for example, normal to the end face 16 of the casting.

The modified layer may be produced in various ways, as by carburizing ornitriding, by casting the bodies in contact with a deposit of thedesired enrichment component, by depleting the layer of a gamma-phasesuppressant, etc., as illustrated by the following examples.

Example 1 Polycrystalline magnets weighing 100 grammes each were cast inthe usual way, of a standard Alnico composition, without any C, Mn orother additions adapted to promote gamma-phase precipitation. Thecastings so produced were divided into two groups.

The magnets of group 1 were heat-treated for a period of 30 minutes atthe temperature of l,0-00 C., for the purpose of obtaining precipitationof gamma-phase, and were then heat-treated for a period of three hoursat the recrystallization temperature of 1,300" C. The treated castingswere then cooled down and crushed. Examination of the crushed castingsshowed that there had been no formation of monocrystals and no granulegrowth.

The magnets of group 2 were covered on all their surfaces, with theexception of one end surface, and were carburized for a period of 30minutes at the temperature of l,000 C., as with standard hardeningprocesses. The carburization of the exposed end surface produced ahardened layer having a depth of hardening about equal to 0.3 mm. and inthis layer produced a carbon content approximately equal to 0.08%. Thecarburization heattreatment also served to precipitate gamma-phase, andno further heat-treatment for this purpose was used.

The magnets of group 2 were subsequently brought to a temperature of1,300 C. and, after diiferent times of stay, were removed from thefurnace, cooled down and crushed. The results were as follows:

Stay for 2 minutes at 1,300 C.: Growth of the granules in the carburizedlayer, no growth of the granules in the non-carburized mass.

Stay for 5 minutes at 1,300 C.: Formation of a large crystal, which haddeveloped in and grown from the carburized layer, and had absorbed intoitself parts of the mass of the magnet casting which had not receivedadded carbon from the carburization.

Stay for 10 minutes at 1,300 C.: A large crystal, which had developed inand grown from the carburized layer, occupied /a of the volume of themagnet;

Stay for 30 minutes at 1,300 C.: Formation of monocrystal in all of thetests.

Example 2 In a plural-cavity sand mold, a thin layer of carbon dust wasdeposited on that surface portion of each cavity which would define anend surface of the magnet body cast therein, and the carbon dust wascompressed to hold it in place. A magnet alloy melt was then poured andcast in the mold in the normal way.

Magnet castings so produced were examined. Microscopic examination ofthe metallurgical structure showed that the castings had absorbed carbonin a thin layer at the surfaces cast in contact with the deposits ofcarbon dust.

The magnet castings produced in this way were heattreated for 30 minutesat a gamma-phase precipitating temperature of 1,000 C., weresubsequently heat-treated for one hour at a recrystallizing temperatureof 1,300 C. and were then cooled and crushed. Examination showed theformation of monocrystals had occurred in all the magnet castings.

Example 3 Magnets of grammes weight cast in normal manner, withpolycrystalline structure, without any addition of C, N, Mn or othergamma-phase precipitant to the standardized composition, were dividedinto two groups.

The magnets of group 1 were at first heat-treated at a tem erature of1,000 C. to promote precipitation of gamma-phase, then heat-treated at arecrystallization temperature of 1,300 C., and then cooled and crushed.The results were as follows: In 19 pieces no formation of monocrystals,in one piece formation of a large crystal that had absorbed about /z ofthe volume of the magnet.

The magnets of group 2 were coated on all surfaces except one end facewith a protective coating, as shown in FIG. 2, and were then subjectedto nitriding in a suitable bath at a temperature of 1,000 C. Thenitriding treatment, which served simultaneously for the precipitationof gamma-phase, lasted 30 minutes.

By microscopic examination, it was found that the nitriding treatmenthad produced a modified layer at the uncoated end face of the castingshaving a depth of about 0.2 mm.

The magnets of group 2 were subsequently heat-treated at a temperatureof 1,300 C., and after different times of stay at that temperature theywere removed from the furnace, cooled down and crushed. The results wereas follows.

Stay for 2 minutes at 1,300 C.: Growth of the crystal granules haddeveloped and progressed from the nitrided layer into the non-nitridedmass of the magnet.

Stay for 8 minutes at 1,3000" C.: Formation of a large crystal that hadalready absorbed about A of the volume of the magnet.

Stay for 25 minutes at 1,300 C.: Formation of monocrystal in all of thetests.

The nitrided layer was removed with the grinder and analyzed. It wasfound that besides the nitrogen absorption in the nitriding bath, also acarburizing effect was produced.

Example 4 As in Example 2, in the cavities of the molds, prior tocasting, there was spread a powdery material, which in this case was notcarbon dust but a fine iron powder with a maximum N content. The magnetsthen were cast in normal manner, then heat-treated as in Example 2, andresearch was carried out to determine the formation of monocrystals.

About 80% of all the samples showed the formation of monocrystals. Inthe remaining 20% of the samples no substantial growth of the granulestook place, and it was found that no nitrogen-enriched layer had beenformed, apparently for the reason that no intimate contact had takenplace between the iron powder with maximum nitrogen content and thealloy of the magnet.

Examples 1 to 4 appear to show that it is by no means necessary to reactthe entire mass of the magnet with elements adapted to precipitate andstabilize the gammaphase (and that, therefore, it is by no meansnecessary to enrich the alloy in its entirety with elements such as C,N, Mn, etc.), but that it is quite suflicient to proceed by enrichmentof a thin layer only with those elements. In that way, there is startedin that layer the formation of monocrystals which propagate further inthe pure alloy of the magnet.

Since the layer in which the added elements have been absorbed isremoved by grinding in the mechanical working which is necessary forother purposes, there is no longer any danger of contamination of thealloy which gives the desired magnetic properties, so that according tothe present invention it is possible to eliminate the cause of dangerstated at the beginning that affects the process described in the priorapplication.

As set forth previously, the elements nickel and cobalt already presentin the alloy also have the effect of Widening the zone of thegamma-phase, and consequently enrichment with these elements also servesto precipitate and stabilize gamma-phase. However, if higher contents ofcobalt or of nickel are included in the whole alloy composition, themagnetic values that can be attained may be reduced below those obtainedwith the standard composition of the alloy. Hence, according to thepresent invention, one proceeds to enrich with nickel or with cobaltonly a thin layer of the mass of the magnet, as was done with additionsof carbon and nitrogen.

8 Example 5 In the cavities of sand molds, prior to casting, were placedsmall discs of pure nickel 0.2 mm. thick and of a size corresponding tothe end face of the magnets. A standard magnet alloy was then cast inthe molds, and the discs became solidly joined to the end faces of themagnet castings.

It was found by microscopic observation of castings so produced that thesmall disc of nickel joined to the end face of the magnet was stillconstituted of nearly pure nickel and that an alloy with the mass of themagnet had been formed for a thickness of only about 0.1 mm. For thatreason, the magnets were brought to a temperature of 1,250 C. in orderto obtain at that temperature an energetic diffusion of the atoms, andin order to realize in that way a better formation of a nickel-enrichedalloy layer with the use of the entire nickel disc.

Subsequently, the magnets were allowed slowly to cool down in a furnaceto favour the segregation of gammaphase, with the result that thecastings remained in the alpha-plus-gamma phase temperature range for aconsiderable period, cooling in 50 minutes time from 1,250 C. down to800 C.

Subsequently, the castings were subjected to recrystallization treatmentfor a period of one hour at a temperature of 1,300 C.

Result: Complete formation of monocrystals in all of the samples.

Example 6 The procedure of Example 5 was accurately followed, with theonly difference that instead of the nickel discs a thin cobalt sheet wasemployed.

Result: Complete formation of monocrystals in all of the samples.

Example 7 The procedure of Example 5 was followed, with the onlyexception that a sheet of commercial type was used having a thickness of0.25 mm., formed of 50% of cobalt and of 50% of iron.

Result: Monocrystals formed in 72% of the castings; in the remainderthere was a vigorous growing of the granules, however, such growth hadnot progressed far enough to form complete monocrystals of the wholecastings.

On the basis of theoretical considerations, it appears that highercontents of nickel or of cobalt have the effect of relatively loweringthe aluminum content, and that it is the relative contents of thesewhich controls gammaphase precipitation and stabilization. A reductionof the aluminum content in the alloy, therefore, ought to act in thesame way to stabilize the gamma-phase, especially since aluminum isknown to be one of the elements that suppresses the formation ofgamma-phase.

It is known that the magnetic characteristics of alnicotype alloys dropvery rapidly with reduction of the aluminum content, and that at analuminum content of 7.6% they attain magnetic values which are too lowto be used. Moreover, the desired degree of stabilizing of gammaphase bya reduction in aluminum content is produced only with values lower than7.4%. In consequence, a reduced aluminum content should be limitedexclusively to a thin layer of the magnet body. The present inventionpermits this to be done.

As an excellent means for the partial reduction of aluminum from thecomposition of a magnet casting, and hence of the formation of a layerpoor in aluminum, oxidation processes proved to be useful. However itwas found very soon that it is not possible to obtain selectiveoxidation of only the aluminum if the temperatures used are too high. Attemperatures of about l,250 C. all of the components of the alloy of themagnet become oxidized and a layer of oxidized material is formed thatis not solidly joined with the mass of the magnet. As a conclusion of along series of researches according to the invention, a method ofoxidation treatment was developed that produces an oxidation of thealuminum only, while it does not involve any appreciable oxidation ofthe other components of the alloy.

Moreover, the course of the temperature relative to time is also used toproduce the desired segregation of gamma-phase to the correct extent.This is illustrated in the following example.

Example 8 Magnets of standardized alloy, cast in normal man ner, werecovered on all their surfaces except one end face with a protectivecoating as shown in FIG. 2. The magnets were then subjected toheat-treatment in a furnace containing normal atmosphere, without anyexcess oxygen and without any protective atmosphere. The cycle ofheat-treatment is shown by FIG. 3 in the accompanying drawings. Thisproduced a decrease in the aluminum content of the magnet alloy at theexposed faces of the magnet castings, sufiicient to stabilize thegammaphase precipitate produced during the early stages of thetreatment, so that such precipitate created the internal strainconditions desired in the subsequent stage of heat-treatment at therecrystallization temperature of 1300 C.

The treatment resulted in the complete formation of a monocrystal in allof the samples.

In all the foregoing examples, it proved to be very convenient andfavorable for the formation of monocrystals to cool the magnets, afterthe heat-treatment for gamma-phase precipitation, to temperatures lowerthan 900 (3., preferably to temperatures of 800 0, prior to therecrystallization treatment. The structure of the gamma-phase in thealpha-plus-gamma temperature range is with cubic reticle with centeredfaces, and lowering of the temperature to below the 900 limit ensuressufiicient cooling to carry the alloy into the alpha-phase temperaturerange, the reticle passes from the cubic with centered faces to become acubic reticle with centered cubes. As is apparent, in passing from onestructure to the other of cubic reticle, by means of the change of thereticle constants, a contribution is made to the increase of theinternal stresses and to the increase of the energy at the boundaries ofthe granules.

Further, in carrying out the invention, it is found that there is aninterdependence between the casting temperature and the certainty ofrealization of the monocrystal. This can be explained by the fact thatas a general rule elevated casting temperatures produce coarsergranules, while lower casting temperatures produce fine granules in thealnico-type alloys. The large granules possess by their nature aquantity of energy at the boundaries of the granules that is relativelysmall and, therefore, they do not display as great a tendency as thesmall granules toward the reduction of energy at the boundaries of thegranules by means of the formation of monocrystals. Lower castingtemperatures and the smaller grain sizes produced thereby are preferred,as is more fully shown by the following example.

Example 9 Casting Probability of Temperature, C.: monocrystals, percent1,460 100 1,550 96 From these results it appears that the castingtempera ture of crude magnets should be as low as possible in practicingthe present invention and preferably should not be higher than themelting temperature increased by C.

I claim as my invention: 1. The process of producing monocrystallinestructure in a normally polycrystalline magnet casting ofiron-nickel-aluminum type permanent-magnet alloy, which comprisesenriching a thin layer of the magnet casting at one face thereof with acomponent which is a gammaphase precipitant and stabilizer, to form insuch layer a composition in which gamma-phase precipitate will form andwill remain stable at recrystallization temperature, subjecting thecasting to substantially uniform heating at a temperature and for a timesufiicient to induce gamma-phase precipitation in said layer in anamount sufficient to produce critical strain in the layer,

subjecting the casting, with the gamma-phase precipitate present in saidlayer to heating at a recrystallization temperature for a timesufiicient to initiate monocrystal growth in said layer,

and continuing such recrystallization heating to cause said monocrystalgrowth to progress from said layer into the main body of the casting.

2. The process as defined in claim 1 in which said layer is enrichedwith a member of the class consisting of carbon, nitrogen, manganese,ruthenium, rhodium, palladium, rhenium, osmium, iridium, platinum, gold,cobalt, and nickel.

3. The process as defined in claim 1 in which said layer is enrichedwith carbon.

4. The process as defined in claim 1 in which said layer is enrichedwith nitrogen.

5. The process as defined in claim 1 in which said layer is enrichedwith manganese.

6. The process as defined in claim 1 in which said layer is enrichedwith cobalt.

7. The process as defined in claim 1 in which said layer is enrichedwith nickel.

8. The process as defined in claim 1 in which said layer is enriched bycasting the magnet alloy into contact with a deposit containing thecomponent which is a gamma-phase precipitant and stabilizer.

9. The process as defined in claim 1 in which said layer is enriched bysubjecting the casting to a surface absorption treatment which diffusessaid component into said layer.

10. The process as defined in claim 9 in which the surface absorptiontreatment is confined to a selected surface portion of the casting.

11. The process as defined in claim 1 in which said layer is enriched bya surface absorption treatment at a temperature and for a time whichboth diffuses said component into said layer and also inducesgamma-phase precipitation in said layer.

12. The process as defined in claim 1 in which the enriched layer isformed by carburizing at a gamma-phase precipitating temperature.

13. The process as defined in claim 1 in which the enriched layer isformed by nitriding at a gamma-phase precipitating temperature.

14. The process as defined in claim 1 in which the magnet alloy containsa gamma-phase precipitating component and sufficient aluminum tosuppress gamma-phase precipitation, and said enriched layer is producedby removing aluminum therefrom to enrich the layer proportionally insaid gamma-phase precipitating component.

15. The process as defined in claim 14 in which the aluminum is removedby oxidation at a low gamma-phase precipitation temperature and suchoxidation is continued for a time to produce sufficient gamma-phaseprecipitate for the subsequent recrystallization step.

16. The process as defined in claim 14 in which the gamma-phaseprecipitating component contained in the magnet alloy is an element ofthe group consisting of co balt and nickel.

17. The process as defined in claim 1 in which the casting is cooled tobelow 900 C. after precipitation of gamma-phase in said layer and priorto said treatment at recrystallization temperature.

18. The process of producing magnets having substantial monocrystallinestructure and of an alloy composition in which the formation ofgamma-phase precipitate is suppressed, which comprises casting saidalloy to form a magnet casting having said alloy and a normallypolycrystalline structure throughout substantially the entire main bodythereof, enriching a thin layer of the magnet casting at one facethereof with a gamma-phase precipitant to form in such layer acomposition in which gamma-phase precipitate will form and will remainstable at recrystallization temperature, subjecting the casting tosubstantially uniform heating at a temperature and for a time sufiicientto induce gamma-phase precipitation in said layer in an amountsufiicient to produce critical strain in the layer,

subjecting the casting, with such precipitate and strain in said layer,to heating at a recrystallization temperature for a time suflicient tocause monocrystal growth to start in said layer,

and continuing said recrystallization heating to cause said monocrystalgrowth to progress substantially into the main body of the casting.

19. The process as defined in claim 18 in which said alloy compositionin an Alnico alloy containing 10 to 30 percent nickel, 6 to 14 percentaluminum, to 42 percent cobalt, up to 8 percent copper, up to 10 percenttitanium, with the balance substantially all iron.

20. The process as defined in claim 19, in which said layer is enrichedwith a gamma-phase precipitant of the group consisting of carbon,nitrogen, manganese, ruthenium, rhodium, palladium, rhenium, osmium,iridium, platinum, gold, cobalt, and nickel.

21. The process as defined in claim 18 in which said casting step iscarried out at a temperature not substantially more than 100 C. abovethe melting temperature of the alloy.

22. The process as defined in claim 18 in which enrichment of said layeris produced by casting the alloy into contact with a deposit containingthe gamma-phase precipitant.

23. The process as defined in claim 22 in which the alloy is cast intocontact with a deposit containing one of the metals of the groupconsisting of nickel, cobalt, and manganese.

24. A permanent magnet casting having a main body portion composed of analloy consisting of 10 to 30 percent nickel, 6 to 14 percent aluminum, 5to 42 percent cobalt, up to 8 percent copper, up to 10 percent titanium,and with the balance substantially all iron, said alloy having acomposition which suppresses the formation of gamma-phase precipitateand in which gamma-phase precipitate is substantially unstable atrecrystallization temperature,

said casting having a thin layer at a limited portion of its surface inwhich the composition of the casting is enriched with a gamma-phaseprecipitant in an amount sufficient to cause gamma-phase precipitate toform and to remain stable at recrystallization temperature, and

said casting having a monocrystalline structure extending through saidlayer and substantially into the main body portion of the casting.

References Cited by the Examiner UNITED STATES PATENTS 1,738,307 12/1929McKeehan 148--12l 2,032,912 3/1936 Corson 148100 2,617,723 11/1952Studders et al 148-101 2,791,517 5/1957 Becker et a1. -123 2,943,0076/1960 Walker et al. l48-1.6 2,970,075 1/ 1961 Grenoble 14831.553,085,036 4/1963 Steinort 14831.57

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,219,495 November 23, 1965 Eberhard Steinort It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1, line 43, for "diection" read direction column 4, line 7, for"30 C." read 930 C. line 25, for "gerat" read great column 6, line 2,for "C, Mn" read C, N, Mn column 7, line 20, for "1,3000 C." read Signedand sealed this 4th day of October 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. THE PROCESS OF PRODUCING MONOCRYSTALLINE STRUCTURE IN A NORMALLYPOLYCRYSTALLINE MAGNET CASTING OF IRON-NICKEL-ALUMINUM TYPEPERMANENT-MAGNET ALLOY, WHICH COMPRISES ENRICHING A THIN LAYER OF THEMAGNET CASTING AT ONE FACE THEREOF WITH A COMPONENT WHICH IS AGAMMAPHASE PRECIPITANT AND STABILIZER, TO FORM IN SUCH LAYER ACOMPOSITION IN WHICH GAMMA-PHASE PRECIPITATE WILL FORM AND WILL REMAINSTABLE AT RECRYSTALLIZATION TEMPERATURE, SUBJECTING THE CASTING TOSUBSTANTIALLY UNIFORM HEATING AT A TEMPERATURE AND FOR A TIME SUFFICIENTTO INDUCE GAMMA-PHASE JPRECIPITATION IN SAID LAYER IN AN AMOUNTSUFFICIENT TO PRODUCE CRITICAL STRAIN IN THE LAYER, SUBJECTING THECASTING, WITH THE GAMMA-PHASE PRECIPITATE PRESENT IN SAID LAYER TOHEATING AT A RECRYSTALLIZATION TEMPERATURE FOR A TIME SUFFICIENT TOINITIATE MONOCRYSTAL GROWTH IN SAID LAYER, AND CONTINUING SUCHRECRYSTALLIZATION HEATINT TO CAUSE SAID MONOCRYSTAL GROWTH TO PROGRESSFROM SAID LAYER INTO THE MAIN BODY OF THE CASTING.